This invention relates in general to mass transfer and heat exchange columns and, more particularly, to methods and apparatus to improve vapor distribution in such columns.
In mass transfer and heat exchange columns, liquid and vapor streams are brought into contact with each other, normally in countercurrent flow, for mass or heat transfer, fractionation or other operations. Various types of internals, such as trays and random and structured packing, have been developed to facilitate interaction between the liquid and vapor streams within selected contact regions of the column. In order to increase the efficiency of the mass transfer or heat exchange taking place between the vapor and liquid within these contact regions, it is important that the liquid and vapor be uniformly distributed across the horizontal cross section of the column, particularly at the lower vapor-liquid interface where the vapor enters the packing or other contacting or internal device.
In columns of the types described above, vapor or mixed phase feed streams are frequently introduced radially or tangentially into the column through a feed nozzle at a location below the contact region. The vapor phase of the feed stream then rises through the contact region and interacts with downwardly flowing liquid. In certain specialized columns, the vapor or mixed phase feed stream is fed at high velocity through the feed nozzle into a flash zone located just above a section where the column transitions to a reduced diameter. The vapor then rises through overlying internals, such as trays, random packing, structured packing, grid packing, open spray chambers or side-to-side shower decks. Examples of such columns include, but are not limited to, virgin crude vacuum columns, virgin crude columns, FCCU main fractionator slurry pumparounds, visbreaker vacuum flashers, heavy oil vacuum towers, heavy oil fractionators, coker main fractionators, visbreaker fractionator, flexicoker main fractionators, and recovered lube oil vacuum towers.
Various devices have been developed in an attempt to interrupt the radial or tangential momentum of the feed stream entering columns of the types described above and redirect it so that it is able to rise in a more uniformly distributed manner across the cross section of the column as well as to separate the liquid components present in the feed stream from the vapor phase. An example of such a device is disclosed in U.S. Pat. No. 5,106,544 to Lee et al., where internal vanes are positioned within an annular vapor horn and are oriented to redirect the vapor or mixed phase feed stream downwardly through the open bottom of the vapor horn. The downwardly deflected vapor is then said to rise in a more uniform manner into an overlying packing bed. These internal vanes are also angled toward the external column shell in the direction of fluid flow so that the feed stream is deflected to impact against the inner surface of the column shell to facilitate separation of the liquid from the feed stream. As a result of computational fluid dynamics (“CFD”) modeling, it has been discovered that the internal vanes, when angled toward the column shell in the direction of fluid flow, create a localized high velocity zone of upwardly flowing vapor in the center of the column. This high velocity zone is undesirable because the high velocity and horizontal maldistribution of vapor reduces the efficiency of the mass transfer or other processing occurring in the overlying zones. A need has thus developed for a way to further improve the distribution of the vapor across the column cross section.